Quantification methods for aeolian sand transport on beaches

Abstract

Quantitative prediction of aeolian sand transport rates on beaches is still a difficult task, mainly due to the large spatio-temporal variability inherent to this type of transport. In order to validate new approaches to calculate aeolian transport, in situ field measurements are needed, combined with the knowledge on how to interpret point measurements in this spatio-temporal varying transport field. Despite the various instruments available and techniques to convert measurements to sand fluxes, there is no consensus about which trapping-device or sensor is the optimal one for aeolian sand transport studies. Often, the results of deployments with electronic sensors (optical and impact sensors) and sand traps are not similar.During the last decade, laser particle counter sensors (Wenglor fork sensors) have been used in various studies to obtain rates of aeolian sand transport in the beach-dune environment. The sensor has been tested in wind tunnels and generally seemed to record aeolian transport properly, and field applications of the sensor reported in literature seemed to provide realistic results. However, some strongly deviating results in our own transport measurements by a co-located sand trap and Wenglor sensor array urged us to further look into the detectability of various grain sizes by the Wenglor sensor.Laboratory experiments were developed, to test the ability of the Wenglor sensor, to accurately count sand grains of various grain size classes and stainless steel beads. It was compared the count data collected by the Wenglor with images from a Highspeed camera which revealed the actual number of grains passing the laser beam. Also, a Silicon photodiode was used to record the laser intensity reduction induced by the sand grain passage through the laser beam to derive the minimal necessary reduction for the Wenglor to count grains, and thereby the minimal detectable grain size. The behaviour of Wenglor laser particle counter was tested in the field. Rates of aeolian sand transport were recorded using Wenglor sensors and co-located vertically stacked mesh sand traps collected sand transported at various elevations. The results show a large variability between fluxes calculated with the sand traps and those derived from the Wenglor counts at the corresponding elevation. The influence of the saturation of the Wenglor in the results was studied and a simulation model of the counting process is presented to look into the role of sediment concentration, sediment fluxes, particle speed and grain size in the mismatch. In the final part of this dissertation an approach for annual scale transport prediction from the intertidal beach is presented. In this approach the surface conditions of the intertidal beach are aggregated, in particular moisture content and roughness. Monitoring data on wind speed, wind direction, rain and water levels is used to calculate the annual onshore aeolian sand transport. To obtain the aggregated value for moisture content the calculated transport for various moisture content values is compared to the volume increase of the dune area obtained from topography. The approach to determine a characteristic moisture content value for aeolian transport gives surface moisture values of 1.2% to 3.2% for wind average and wind gust respectively, implying that to achieve the dune volume change a quite dry beach is necessary. This indicates that the main area for aeolian transport corresponds to the upper part of the intertidal source, most likely the region between mean high tide line and spring high tide line.